(i) Field of the Invention
The present invention relates to an ink for a ball point pen and a type of a ball point pen which contains the ink in an ink tank or tube.
(ii) Description of the Related Art
Ball point pens typically comprise a pen point comprising a ball and a tip holder, an ink reserving tube, a penholder and the like. The ball of the pen point is rotated to transfer or sink an ink onto or into a recording medium such as a paper. In this way, written traces drawn lines can be made. During writing, excessive ink which has not been transferred or sunk to the recording medium adheres to the outer periphery of the tip holder, so that a blobbing phenomenon occurs where ink drops form at the start of writing or during writing, with the result that sharp lines sometimes cannot be drawn.
In order to solve this problem, for example, Japanese Patent Application Laid-open Sho No. 5-40192 has disclosed a method in which the outer periphery of the tip holder is coated with a water-repellent or an oil-repellent material so as to inhibit the ink from adhering to the outer periphery of the tip holder.
However, the coating material is peeled off by friction with a writing material such as paper during writing, resulting in the gradual onset of the blobbing phenomenon.
The principle by which the coating works is that the critical surface tension on the outer periphery of the tip holder is reduced so as to be lower than the surface tension of the ink, thereby decreasing the ability of the ink to wet the tip holder. However, since inks tend to have low surface tension, the surface of high-energy materials such as metal are disadvantageously wetted with ink. Because of this, it is easier to wet metal with a non-aqueous ink than an aqueous ink.
Even if the above-mentioned coating is applied to the surface of a low-energy material, there is only a slight difference between the critical surface tension of the outer periphery of the tip holder and the surface tension of the ink, and hence the oil-repellent effect of the coating is too poor to sufficiently prevent ink blobbing.
Furthermore, in the usual non-aqueous ink-containing ball point pen, the tip wear due to the rotation of the ball is relatively low, because the ink has a viscosity of thousands to millions of cps. However, if the viscosity of the ink is less than thousands of cps, lubrication of the ball by the ink does not occur, so that the tip becomes extremely worn, with the result that ink flow decreases and in some cases stops.
In a conventional non-aqueous ink-containing ball point pen, the ink contains a solvent having a low vapor pressure as a main component, and therefore, even if the cap is removed and the pen point is exposed to air, ink-flow decreases or cessation rarely occurs. However, since the non-aqueous ink has a high viscosity, of thousands to about one million mPa.multidot.s, a high writing pressure is required. Consequently, the fingers, arm and shoulder of a writer are easily tired by writing for a long period of time with a conventional non-aqueous ink-containing ball point pen. On the other hand, in an aqueous ink-containing ball point pen, the ink has a low viscosity, and therefore the writing feeling is light. However, the main solvent is water which easily vaporizes. Therefore, if the cap is removed and the pen point is exposed to air, decreases in or cessation of ink flow frequently occur.
Japanese Patent Application Laid-open Hei No. 1-299880 discloses a non-aqueous ink for a ball point pen in which the surface tension of the organic solvent is regulated and which has an ink viscosity of 50 to 2000 cP (25.degree. C.) and possesses some of the characteristics of a non-aqueous ink and an aqueous ink, but this kind of ink is not always satisfactory. In recent years, some aqueous inks having an intermediate viscosity are on the market as inks having many advantageous features with respect to quality and economy, but these inks are inferior to non-aqueous inks, in cap-off performance, in that ink flow cessation rarely occurs with non-aqueous inks even when the cap is removed.